1. Field of the Invention
The present invention relates to an inkjet recording ink (hereinafter may be simply referred to as “ink”), and an ink cartridge using the ink, an inkjet recording apparatus, an inkjet recording method and an ink recorded matter.
2. Description of the Related Art
In recent years, as an image forming method, an inkjet recording method has come into wide use because the inkjet recording method is simpler in process and easier in printing in full color mode as compared to other methods, and has an advantage such that high resolution images can be obtained even with apparatuses simple in constitution. The inkjet recording method is such that small quantities of ink are allowed to fly with an inkjet recording apparatus and to attach onto a recording medium such as paper so as to form an image; the use of the inkjet recording method has expanded so as to cover personal and industrial printers and printing.
In an inkjet recording apparatus, aqueous inks using water-soluble dyes as coloring agents are used, and the dye inks involve drawbacks of being poor in weather resistance and water resistance. Accordingly, recently, there have been advanced researches on pigment inks using pigments in place of water-soluble dyes. However, the pigment inks are, as compared to dye inks, still poorer in terms of color developing property, ink ejection stability and ink storage stability. The improvement of the high image quality achieving technique of OA printers requires pigment inks to give image densities comparable with the image densities given by dye inks even for plain paper as a recording medium. However, when plain paper is used as recording media, pigment inks penetrate into the sheets of paper so as to lower the image density on the surface of the sheets of paper, and thus cause a problem such that the image density is lowered. For the purpose of increasing the drying rate of the ink adhering onto a recording medium in order to achieve high speed printing, a technique is adopted in which a penetrating agent is added to the ink to facilitate the penetration of the water in the ink into the recording medium so as to promote the drying of the ink; however, in this case, the penetrability of the pigment as well as the penetrability of water is enhanced to cause a phenomenon of further decreasing the image density.
For the improvement of the image density, various methods have been proposed. For example, there has been proposed an aqueous inkjet recording ink which includes, as essential components, carbon black or an organic pigment, a polymer, a basic substance and water (see, Japanese Patent Application Laid-Open (JP-A) Na 2007-153986). The ink based on this proposal is characterized by including a copolymer prepared by polymerizing, as essential components, a (meth)acrylic acid and another copolymerizable monoethylenic unsaturated monomer, and a copolymer prepared by copolymerizing, as essential components, a phosphate group-containing monoethylenic unsaturated monomer and another copolymerizable monoethylenic unsaturated monomer.
According to the method of Japanese Patent Application Laid-Open (JP-A) No. 2007-153985, when the content of the phosphate group-containing monoethylenic unsaturated monomer in the copolymer is low, no sufficient image density is obtained. When the content of the phosphate group-containing monoethylenic unsaturated monomer in the copolymer is increased, the image density is improved, but the dispersion condition of carbon black or the organic pigment in the ink comes to be unstable. No compatibility is achieved between the high image density and the dispersion stability of the pigment as a pigment dispersion and a pigment ink.
Alternatively, an ink has been proposed which is used for recording on paper, including a water-soluble multivalent metal salt (Japanese Patent Application Laid-Open (JP-A) No. 2011-122072). The ink based on this proposal include a pigment (a) and at least a compound (b) having no surface activity, having a molecular weight of 150 or more and 10,000 or less, and having a functional group selected from a functional group whose basic skeleton is phosphoric acid and a functional group whose basic skeleton is phosphonic acid wherein the content of phosphorus ((P content/molecular weight)×100) derived from the functional group whose basic skeleton is phosphoric acid or the functional group whose basic skeleton is phosphonic acid in the molecular structure of the compound is 1.4 or more, and the content of the (b) compound is 1.5% by mass or more and 10.0% by mass or less in relation to the total mass of the ink.
However, the method of the foregoing proposal results in an insufficient improvement of the image density for plain paper having a low content of the water-soluble multivalent metal salt. The use of the compound having the functional group selected from the functional group whose basic skeleton is phosphoric acid and the functional group whose basic skeleton is phosphonic acid improves the image density, but causes a problem such that the pigment dispersion and the dispersion condition of the pigment in the ink come to be unstable.
Accordingly, it is desired to provide an inkjet recording ink, capable of achieving the compatibility between the high image density on plain paper and the improvement of the storage stability of the pigment dispersion and the ink.